This invention relates to a system and method for rapidly sorting irregularly shaped scrap metal particles randomly located on a moving conveyor based on the optical analysis of laser-induced plasmas.
The spectroscopy technique known as Laser-Induced Breakdown Spectroscopy (LIBS), Laser Spark Spectroscopy (LSS), or Laser-Induced Optical Emission Spectroscopy (LIOES) uses a focused laser beam to vaporize and subsequently produce spectral line emissions from a sample material. In this way samples placed at a distance from the analyzing instrumentation, can be analyzed for their chemical composition. It has been shown that a plurality of laser pulses increases the sensitivity of the technique for samples contaminated with paint, dirt, etc., on their surface. Since objects located at a distance could be analyzed rapidly, the method has also been recommended as a possible detection technique in the rapid sorting of mixed scrap metals (see xe2x80x9cAnalysis of Metals at a Distance Using Laser-Induced Breakdown Spectroscopy.xe2x80x9d D. A. Cremersxe2x80x94Appl. Spectroscopy 1987).
Attempts have been made to use laser-induced breakdown spectroscopy (xe2x80x9cLIBSxe2x80x9d) to analyze metal particles to determine their composition in order to sort the particles. U.S. Pat. No. 5,042,947 discloses utilizing LIBS to sort metal particles. The ""947 patent, however, requires the use of a randomly triggerable laser system, such as an excimer laser. Excimer lasers are generally more expensive and less industrially rugged compared to fixed frequency solid-state lasers such as Nd-YAG lasers.
The ""947 patent also requires that the metal particles be conveyed into an inspection path where they will each individually pass a fixed inspection point upon which the pulsed laser beam is trained. This process, however, provides for no mechanical means for arranging the moving scrap metal particles in single file, or evenly spacing the particles in a manner expected by the fixed inspection and analysis instrumentation. Experience shows that it is difficult, if not impossible, to arrange and evenly space scrap particles in a high volume production environment. Even if this is possible, the speed of processing would be greatly limited by the speed of orienting each of the scrap particles into a single file so that each of the particles passes the fixed inspection point.
The ""947 patent further requires irradiation of the surface of each particle with an initial cleaning pulse and subsequent removal of the plasma produced by the cleaning pulse using an air jet prior to the irradiation of the surface of the particle with an analyzing pulse. An air jet requires a finite time to initiate, and a rapidly moving particle would advance a certain distance during this time. Thus, it would be difficult, if not impossible, to direct more than one pulse to the same target spot of a selected particle. In general, because of the irregular shape of scrap metal particles, it would be difficult, if not impossible, to strike a particle on the same spot when the laser beam is redirected between pulses. Again, even if practically feasible, these additional processing steps would greatly reduce the speed of processing the scrap particles.
It is also known to sort objects located on a moving conveyor by periodically acquiring color images of the objects and discriminating between the objects on the basis of their color. While this method may be employed for sorting certain metal scrap, such as, for example, sorting copper from aluminum, image processing-based color sorting systems are ineffective for sorting randomly oriented scrap metal particles of similar colors, such as aluminum and magnesium, or different aluminum alloys.
It is particularly desirable to efficiently separate scrap into alloy families, since mixed scrap of the same alloy family is worth much more than that of indiscriminately mixed alloys. For example, in the blending methods used to recycle aluminum, any quantity of scrap comprised of similar alloys and of consistent quality, has more value than scrap consisting of mixed aluminum alloys.
One object of the present invention is to provide a system for sorting irregularly shaped scrap metal particles based upon the optical analysis of laser-induced plasmas, as the particles are rapidly transported in random locations on a conveyor.
Another object of the present invention is to provide a system which is capable of rapidly sorting metal scrap particles, such as different aluminum alloy families, which may not be easily differentiated from each other by their color.
Another object of the present invention to provide a LIBS metal scrap sorting system including a scanner system which provides focused laser pulses having uniform power densities along a plane located at the surface of the conveyor.
Another object of the present invention is to provide a LIBS metal scrap sorting system including image detection and processing arrangement that could be used to provide information about a plurality of particles randomly located on a conveyor belt, including location, size, and shape of each particle, to a sorting control system.
Another object of the present invention is to provide a LIBS metal scrap sorting system including an illumination system for image detection that provides uniform controlled illumination of the scrap pieces from a plurality of directions so that the image detected is an actual two-dimensional outline of the particle and is not influenced by the combined effects of particle shape, surface reflectivity, and the geometry of the illumination system.
Another object of the present invention is to provide a LIBS metal scrap sorting system including a fixed frequency laser system configured so as to frequently provide multiple laser pulses during an extremely small interval of time so that all the pulses could be directed to a randomly located scrap particle using a single positioning movement of a laser beam scanner.
In carrying out the above and other objects, the scrap sorting system of the present invention includes a conveyor for conveying the randomly shaped scrap metal particles in a random orientation, an image detector for electronically recording the image of a predefined viewing area through which the scrap particles are conveyed by the conveyor, a position detector for detecting movement of the conveyor, a laser system configured to provide a laser beam including a stream of a plurality of laser pulses within a selected time interval, and at least one laser scanner assembly including a positionable beam deflector to direct the laser pulses at a selected particle at any location in a selected target region on the conveyor and a focusing element mounted downstream of the source of the laser pulses from the beam deflector to focus the beam and provide uniform laser power density along a plane. The system further includes a light collector for collecting light from plasma produced from the particles as they are irradiated by the laser pulses, a light distribution and spectral analyzer system for isolating and measuring at least one selected band from the collected light, a separator to divert particles to different bins based on discriminator signals, and control logic for continuously acquiring an image of the selected viewing area of the conveyor, processing the image to identify and locate the scrap particles as they pass through the viewing area, monitoring the laser system to determine when the next laser pulses will be available, selecting a downstream location on the conveyor at which the next available stream of pulses of radiation may be directed at an identified particle, operating the scanner assemblies as required to direct the pulses at the selected target location, analyzing spectral data collected from the plasma, generating a discriminator signal based at least in part upon the spectral data analysis, and selectably activating the separator as a function of the discriminator signal to sort the analyzed particles.
In one embodiment, the laser beam is positionable both in the direction of travel of the conveyor as well as transverse to the direction of conveyance (i.e. across the width of the conveyor), so that a downstream location can be selected for irradiating each particle regardless of that particle""s location on the moving conveyor. A target scanner assembly comprising one or more beam deflectors are utilized, each capable of directing multiple laser pulses when available to a selected point within a target area on the conveyor in time to ablate a scrap particle as it passes that point in the scanner""s target area. The beam deflectors preferably comprise one or more galvanometric scanners including positionable mirrors for controllably directing the laser beams.
It will be appreciated that enough target scanner assemblies are employed so that the combined target areas of each of the scanner assemblies covers the entire width of the conveyor belt. In addition, some overlap of the target areas in the direction of the width of the belt may be desirable, for example, in an orientation where the target area of one scanner assembly is located downstream in the direction of travel of the conveyor but overlapping another target area in the direction of the width of the conveyor, so that alternative target points are available for a particular identified piece of scrap. This overlapping coverage of the conveyor using multiple scanner assemblies could provide for faster processing of the particles, either by allowing for a higher density of particles on the conveyor, or for processing at a higher conveyor speed.
In one embodiment, the system employs a laser system including two solid-state, fixed frequency lasers. It is desirable to provide a plurality of pulses to a selected target point within a period sufficiently short enough to allow for each of the laser pulses to strike and ablate a selected scrap particle at the same spot as it is being conveyed past that target point. Thus, for example, on a conveyor that is moving at approximately 300 feet per minute, the desired number of pulses must be fired and directed to the target point to strike the particle within a period of approximately 250 microseconds. For this time period, the movement (approximately 0.38 millimeters) of the scrap particle can be ignored considering the spot size of the laser beam, and the particle may be ablated with multiple pulses without operating the scanner assembly to redirect the laser beam between pulses.
In the preferred embodiment, up to four pulses may be directed on a target within 150 microseconds using two constant repetition rate Nd-YAG lasers operated in xe2x80x9cdouble pulsexe2x80x9d mode. In the double-pulse mode, the laser is Q-switched twice during a flashlamp cycle, resulting in two pulses separated by an extremely small time period (approximately 1-200 microseconds). Since particle movement during this time period is negligible, all four shots could be directed to the same spot using a single positioning of the scanner.
In yet another embodiment, a plurality of laser systems are provided, with each system capable of providing the desired plurality of pulses within a time interval sufficiently short to ablate a target point on a selected particle without repositioning the scanners between pulses. It will be appreciated that, by pooling a plurality of laser systems, the number of particles that can be sorted per second is not dependent upon the frequency of generation of the pulse stream from any single laser system.
An image detector, preferably including a conventional linescan camera, is located upstream from the scanner target area(s) to continually acquire images of a suitably illuminated image viewing area, and to determine the existence and location of scrap particles on the moving conveyor. The image detector also preferably includes a lighting system to provide controlled, uniform illumination of the viewing area.
A light collector, preferably comprising a plurality of optical fibers, is mounted to receive light from the generated plasma in the target area(s). A light distribution and spectral analyzer system is also provided for isolating and measuring at least one selected band from the light collected by the optical fibers.
The light distribution and spectral analyzer system includes at least one spectral filter, preferably in the form of a monochromater for isolating a selected band from the collected light, and a detector associated with each monochromater for generating a signal corresponding to the intensity of the selected band.
A system control, preferably in the form of one or more suitably programmed computers, monitors and manages collection of system data, including conveyor belt position data received from the position detector, image data received from the camera, laser pulse availability, and spectral emission data received from the light distribution and spectral analyzer system, and processes the data to control all of the major components of the system.
The system control also includes image processing logic for detecting the presence and location of the scrap particles, and may include additional known image processing capabilities, such as known shape and color information extraction algorithms, which may be utilized for sorting the scrap particles. Once identified, the linear advancement of these particles on the conveyor belt is monitored by the control system based on belt position data supplied by an encoder. When the control system determines that the next laser pulse stream will become available at a predetermined time in the future, the system identifies a particle in a target area. The scanner mirrors of the appropriate scanner assembly are positioned to deflect the pulse stream to a position expected to be occupied by the particle when the laser pulses become available.
The system control also includes data acquisition logic suitable for acquiring data from the various input hardware components, decision logic such as neural networks capable of classifying each of the scrap particles as one of a preselected list of alloy families based on the analysis of spectral and other data, output control logic for control of output hardware, and networking logic for the interconnection and the seamless operation of the various computer systems.
The system of the present invention thus effectively sorts randomly shaped and randomly located scrap metal particles, based on their chemical composition as they are conveyed on a high speed conveyor.
These and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.